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Fire coat prevention on castings

 Do you have fire coat on your silver castings? 

What is fire coat? �The scourge of every silversmith who works with
sterling �is the dark copperiest purple that forms on sterling when
it is heated above about 1000 degrees. ��I shows up when the piece is
polished. Upon heating, oxygen forms with the copper in the sterling
to form fire coat. �The longer the silver is heated the deeper the
fire coat is formed. ��

Fire coat can not be removed by the normal pickling process. ��It
must be removed by reverse plating (stripping) or by abrasion.
��Unfortunately abrasion removes some of the detail. ��Both options
presents problems

Cast sterling silver contains a deep layer of fire coat because of
the time the silver is exposed to oxygen as it cools in the mold.

I have a very simple technique to prevent fire coat when vacuum
casting. �The idea is to remove the oxygen from the silver as it

Before I pour the metal I place some wax shavings on a solid
soldering pad . I select an empty flask that is the next size larger
than the one I will be pouring the silver into.

I proceed to cast as usual. �As soon as the metal is poured I remove
the mold flask from the vacuum table and place it on the wax covered
pad. ��I then place the empty flask around the mold flask. �I then
throw some wax �on the sprue button. �I immediately place a solid
soldering pad on the top of the empty flask. �Note: �the wax will
flash. �

I �quench the mold flask about a minute or more later that I normally
would. �This is because the covered mold flask does not cool as fast
as it would if exposed.

This process virtually eliminates oxygen from the silver during the
cooling process thereby reducing the finishing time. �I have taken my
castings from the quench water and polished them on my jewelers rouge

Note: When black castings are placed in the pickle pot, after
quenching to remove the oxides, �the surface of the silver is
slightly etched. �The silver will have a slight frosted appearance.
��My castings come out dull to bright silver and as smooth as the
original wax. ��I do not pickle them unless I have to perform a
solder task.

The theory is that the wax inside the cover flask is attempting to
burn. �This produces a reducing atmosphere inside the empty flask.
�There is no oxygen left to combine with the copper in the silver. ��

It is important to place the mold flask into the empty flask as
quickly as possible. �Fire coat is forming as long as the silver is
exposed to oxygen.

Note: �The process is more fully described in the June 1995 issue of
Rock and Gem magazine and the December 1997 issue of Lapidary

Some of my work is shown in Ganoksin. �I have been vacuum casting
with a handy melt furnace since 1975. Lee Eppersom